Thanks for signing up! You might also like these other newsletters:

What it screens for: The Rh factor is a protein carried by red blood cells in some people, and not in others. If you have the protein, you are Rh positive. If not, you are Rh negative. (And you are special: Only about 15 percent of the population is Rh negative.)

In blood typing, everyone is either type A, B, or O; the plus or minus sign after the letter refers to the Rh factor. Both Rh negative and Rh positive are entirely normal, healthy blood characteristics. Problems can arise, however, if an Rh-negative mom-to-be carries an Rh-positive baby. The mother's body may mistake the baby's blood cells as intruders and start making antibodies to attack them. Left unchecked, this condition (known as fetal Rh disease) can threaten the health of the baby. This almost never happens in a first pregnancy (since the baby's blood is unlikely to enter the mom's bloodstream until delivery). However, if untreated in the first pregnancy, it can threaten subsequent pregnancies. So as a preventive measure, all Rh-negative women are given injections of a substance called RhoGAM (Rh-d immune globulin, which prevents the antibodies from forming) at various times during each pregnancy — starting with the first. These injections save the lives of an estimated 10,000 babies per year in the United States alone.

Who it's for: All pregnant women will have their Rh factor determined. Rh-negative woman will have follow-up testing and treatment.

How it's done: Blood samples are taken from a vein in your arm. If you are Rh-negative, a RhoGAM injection goes into your muscle tissue in your arm or your backside. You might be given a choice; or your practitioner might favor one spot or the other. The injection is somewhat painful and the soreness can last for a couple of days. Ask your practitioner about taking a pain reliever to alleviate the discomfort.

When it's done: Rh testing is usually done during the initial blood workup. RhoGAM injections for Rh-negative women are given at 28 or 29 weeks and again within 72 hours of delivery. The RhoGAM injection is also administered after any genetic testing that could result in mixing of maternal and fetal blood, such as CVS ( chorionic villus sampling) or amniocentesis. Spotting, miscarriage, and abortion are the other situations where fetal blood can get into a pregnant woman's bloodstream, so RhoGAM is given to those who are Rh-negative after these events as well.

Risks: There is little or no risk associated with blood tests. Note: If you are Rh-negative, the risk does go up with every subsequent pregnancy (as your body builds more and more antibodies). Fortunately, thanks to the widespread use of this screening test and safe, effective treatment, fetal Rh disease is now very rare.